Power inverters convert a DC power to an AC power. Some power inverters are configured to convert the DC power to an AC power suitable for supplying energy to an AC grid and, in some cases, an AC load coupled to the AC grid. One particular application for such power inverters is the conversion of DC power generated by an alternative energy source, such as photovoltaic cells (“PV cells” or “solar cells”), fuel cells, DC wind turbine, DC water turbine, and other DC power sources, to a single-phase AC power for delivery to the AC grid at the grid frequency.
The amount of power that can be delivered by certain alternative energy sources, such as photovoltaic cells (“PV cells” or “solar cells”), may vary in magnitude over time due to temporal variations in operating conditions. For example, the output of a typical PV cell will vary as a function of variations in sunlight intensity, angle of incidence of sunlight, ambient temperature and other factors. Additionally, photovoltaic cells have a single operating point at which the values of the current and voltage of the cell result in a maximum power output. This “maximum power point” (“MPP”) is a function of environmental variables, including light intensity and temperature. Inverters for photovoltaic systems typically comprise some form of maximum power point tracking (“MPPT”) as a means of finding and tracking the maximum power point (“MPP”) and adjusting the inverter to exploit the full power capacity of the cell at the MPP.
An important parameter used to measure the performance of alternative energy source inverters is the efficiency of the inverter. Efficiency is typically defined as the ratio of output power from the inverter to input power to the inverter. Although at first glance, improvement of efficiency appears to be a straightforward, improving or otherwise controlling the efficiency of alternative energy source inverters can be complicated. Such complications occur because the efficiency of the inverter may vary with the output power from the inverter (e.g., the efficiency may decrease as the output power decreases). Additionally, some energy efficiency measurement protocols weight the efficiencies of inverters measurements based on the percentage of the rated power. For example, some measurement protocols apply a significant weight to the efficiency of the inverter at light loads, which as discussed above may be at the inverter's lower efficiency end. Alternatively, other energy efficiency measurement protocols utilize a “flat” weighting curve. However, even under such alternative measurement protocols, many inverters naturally exhibit a lower efficiency at lower output power levels.
To improve performance under such efficiency measurement protocols, some inverters implement a “burst mode” technique in which the output of the inverter is turned on for short durations to periodically generate one or more full sinewave output cycles. For example, as shown in FIG. 13, such “burst mode” inverters may skip one or more output cycles such that a full sinewave output cycle is generated only every other cycle. However, the “burst mode” technique may be restricted to use only at very light loads due to regulatory requirements, can cause significant harmonic distortion, including subharmonics, in the inverter output, and may be difficult to implement in an array of individual inverters and/or alongside a standard run mode of the inverter.